The Milky Way seems to be gobbling up a mysterious amount of gas—more than it spits back out, according to a new paper.
Our galaxy maintains a level of “stuff” through a feedback process: It eats up gas from the surrounding environment to form stars, and then it spits the gas back out via supernovas and stellar winds. Using data from the Hubble Space Telescope, a team of astronomers calculated that more mass appears to be flowing in than is flowing out—an unexpected result, according to a Hubble press release.
The researchers collected data from 270 views taken by the Cosmic Origins Spectrograph instrument on the Hubble Space Telescope. Specifically, they were interested in speedy clouds of gas that moved too quickly to be orbiting with the Milky Way. These clouds appear as features that absorb ultraviolet light passing from behind them, like shadows on specific light wavelengths. Doppler shifts—the clouds absorbing longer or shorter wavelengths than expected—allow the researchers to determine whether the clouds are flowing out or in. They found 187 of these clouds.
When the researchers calculated the rate that mass was flowing into the galaxy versus out of it, they found the scales tipped—the galaxy sucks up more gas than it spits out, according to the paper published in the Astrophysical Journal. The Milky Way could simply be in an inflow-dominated part of its life, the researchers write in the paper, but the presence of mass moving in either direction lends credence to ideas that the galaxy used gas from its surrounding environment to produce stars, then recycles it back into the environment once the stars die.
So, where’s the extra mass coming from? Study author Andrew Fox from the Space Telescope Science Institute suggested that perhaps the Milky Way was slurping up mass from the intergalactic medium, as well as its smaller satellite galaxies, according to the release.
The authors note that the calculations are based on solely a snapshot of the Milky Way today, and that caution should be taken when trying to extrapolate the rates that mass is flowing over time, according to the paper. Additionally, the ultraviolet absorption by these high-velocity clouds is just one way to measure the rate that the galaxy is eating and spitting out gas. These measurements present a higher rate of gas flowing inwards than radio-based measurements. Plus, the authors write, they only looked at fast-moving clouds; maybe the slow-moving clouds would reveal a different story.
Ultimately, though a mystery is exciting, studies like these serve a more profound purpose: to understand how our galaxy and others fit into the grander scheme of the universe, and how they get the necessary material to form stars and, in turn, planets.